The invention relates to a process for preparing a high-molecular polyamide, polyester or polyester-amide block copolymer by melt-mixing polyamide or a polyester or a mixture of a polyamide and a polyester having a lower molecular weight with a bislactam.
Such a process is for example known from EP-A-0288253, in which, as in other publications, use is made of bis-N-acyl lactams having the formula: 
in which A=alkyl or an aromatic group and n is generally between 3 and 11.
The bis-N-acyl lactams used in the examples are generally tere- or isophthaloyl bislaurocaprolactam or biscaprolactam.
These bis-N-acyl lactams however have the drawback of a relatively low reaction rate, as a result of which long reaction times are required to realize the desired increase in molecular weight, which may lead to undesired side-reactions, e.g. discolouration of the polyamide or polyester.
In practice, bisoxazolines or bisoxazines are therefore more preferably used as chain extenders for polyamides. The drawback of these is however that they react with the polyamide""s carboxylic end groups, as a result of which the polyamide obtained contains excess amino end groups and the polyester excess hydroxyl endgroups, which adversely affects the thermal oxidative stability.
The invention""s aim is hence a process in which a bifunctional chain extender that does not involve the above drawbacks is added to a polyamide or a polyester melt.
The inventors have now most surprisingly found that when the polyamide or the polyester having the lower molecular weight reacts in the melt with a carbonyl bislactam, a colourless, stable polyamide or polyester with increased molecular weight is very quickly obtained.
xe2x80x98Carbonyl bislactamxe2x80x99 is understood to be a compound having the formula: 
in which n is an integer of between 3 and 15. Preferably n=5 to 12.
The carbonyl bislactam can be obtained in a simple manner through reaction of the lactam with phosgene, COCl2. The preparation of N,Nxe2x80x2-carbonyl biscaprolactam via this route in benzene in the presence of a tertiary alkylamine as a catalyst is for example described in JP-A-42017832.
The amount of carbonyl bislactam used in the process according to the invention may vary within a wide range. Usually at least about 0.1 wt. %, relative to the polyamide or the polyester, will be required to obtain an appreciable effect. Amounts of more than 4 wt. % do not usually lead to a further increase in the molecular weight.
Usually a person skilled in the art will adjust the amount of carbonyl bislactam to be used to the number of amino or hydroxyl end groups available and the increase in viscosity to be realized as a result of the increased molecular weight. He will usually determine the optimum amount for his situation through simple experimentation.
In principle, the process of the invention can be used for all types of polyamides and polyesters. The polyamides include at least the aliphatic polyamides, for example polyamide-4, polyamide-6, polyamide-8 etc., polyamide-4,6, polyamide-6,6, polyamide-6,10, etc., polyamides derived from an aliphatic diamine and an aromatic dicarboxylic acid, for example polyamide-4,T, polyamide-6,T, polyamide-4,I, etc., in which T stands for terephthalate and I for isophthalate, copolyamides of linear polyamides and copolyamides of an aliphatic and a partially aromatic polyamide, for example 6/6,T, 6/6,6/6,T, etc. The process is particularly advantageous in the case of partially aromatic polyamides and copolyamides that require in general a relatively long polymerisation time.
The polyesters include at least polyesters derived from aliphatic dicarboxylic acids and dials, polyesters from aliphatic dials and aromatic dicarboxylic acids, copolyesters that are partially aliphatic and partially aromatic and polyesters that contain units derived from cycloaliphatic dicarboxylic acids. Specific examples are polybutyleen adipaat, polyethyleen terephtalate, polyethyleen naphtalate, polybutyleenterephtalate, copolyesters of polybutyleenadipate and polybutyleenterephtalate, the polyester derived from butanediol and cyclohexane dicarboxylic acid.
The process according to the invention can be carried out in a simple manner using the usual melt-mixing techniques and equipment, for example by dry blending the polyamide or polyester having a lower molecular weight and the bislactam and optionally also other additives in a solid state, for example in a tumbler drier, after which the mixture obtained is melted in a usual melt-mixing apparatus, for example a Haake kneader, a Brabender mixer or a single- or double-screw extruder. The different components can also be fed to the mixing apparatus separately.
Best results are obtained if the lower molecular polyamide or polyester are thoroughly dried.
Preferably the carbonyl bislactam is added to the melted polyamide or polyester product stream in the polymerisation process as it leaves the polymerisation reactor. The polymerisation process can be carried out both batchwise or in a continuous mode. In the first case a reduction of the residence time in the reactor can be realized and thus an increase in productivity; with the continuous process the after-condensation step, that is necessary usually to obtain a polyamide or polyester of sufficient molecular weight can be avoided.